In an X-ray CT apparatus, while both an X-ray source and an X-ray detector positioned opposite to the X-ray source are rotated around an object to be examined, X-ray beams (fan beams) are irradiated from the X-ray source in a fan shape, and then, an amount of X-rays which have penetrated through the object is detected by the X-ray detector. Transmission X-ray amount data detected by the X-ray detector is acquired, and a tomographic image is reconstructed based on this acquired data by a reconstructing means.
A fan-shaped X-ray beam owns a certain extended projection angle and a certain width. This width will be referred to as a “beam slice width.” Since a width of an X-ray detector is made equal to this beam slice width, a slice width of a tomographic image is made coincident with the beam slice width.
Conventionally, in order to obtain a tomographic image having a slice width which is thinner than a width of an X-ray detector, a slice width adjusting means is employed. As this slice width adjusting means, since a collimator for limiting an irradiation range of X-rays along a thickness direction thereof may be installed, or a mechanism for adjusting a detection range of the X-ray detector along a width direction thereof may be provided, such a method for limiting an X-ray incident plane has been conducted.
As previously described, the conventional method owns such a drawback that the adjusting means should be provided.